https://codepen.io/werenall-the-lessful/pen/byBRXo

central-element.css

.canvas {
  width: 600px;
  display: flex;
}
.canvas__left {
  display: flex;
  flex: 1;
  flex-shrink: 0;
}
.canvas__right {
  display: flex;
  flex: 1;
  justify-content: flex-end;
}
.canvas__center {
  //flex-shrink: 0;
}

/* The following styling is just for the purpose of making this example more legible. You can ignore it.*/
.canvas__left {
  background: cyan;
  border: solid 5px cyan;
}
.canvas__right {
  background: cornsilk;
  border: solid 5px cornsilk;
}
.canvas__center {
  background: tomato;
  border: solid 5px tomato;
}

central-element.html

<div class="canvas">
  <div class="canvas__left">
    <button>a</button>
    <button>b</button>
    <button>c</button>
  </div>
  
  <div class="canvas__center">
    <div>I'll try my best to be in the centre</div>
  </div>
  
  <div class="canvas__right">
    <button>1</button>
    <button>2</button>
    Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat
  </div>
</div>